In the same way it's an improvement over other Lisp dialects. There's no huge hole in Arc that Bel fixes. Just a lot of things that are weaker or more awkward or more complicated than they should be.
> The code I wrote to generate and test the Bel source is written in Arc
Was this during bootstrapping, or is this still the case? Or in other words, do you now edit bel.bel directly or are there arc files you edit that compile into bel.bel?
It's still the case. bel.bel is generated by software. Most of the actual code is Arc that I turn into Bel in generating it. E.g. the interpreter and anything called by it, and the reader. But code that's only used in Bel programs, rather than to interpret or read Bel programs, can be and is written in Bel.
I had to change Arc a fair amount to make this work.
Curiously, enough, though, I found doing development in Bel was sufficiently better that I'd often edit code in bel.bel, then paste a translated version into the file of Arc code, rather than doing development in the latter. This seemed a good sign.
